013 Observing Reactions

Expt 013 -- Observing Reactions

This experiment is intended to strengthen observational skills. A set of unknown chemicals is provided. The objective is to mix these chemicals, two at a time, and observe evidence for reactions.

Background

What is the best way for observations to be checked? Scientists perform experiments, draw conclusions, and use the scientific literature to report their experiments and conclusions. How does this process work? The purpose of this experiment is to make observations and decide how best to deal with handling those observations.

Safety

Several of the chemicals are toxic. The vapors of several chemicals are toxic. Wear goggles and apron. Unknown chemicals may pose unexpected hazards, so use caution in dealing with them. Wash spills with water. Wash hands immediately upon contact with chemicals. Avoid breathing vapors from chemicals. Avoid ingesting the chemicals.

Procedure

Obtain a reaction grid from the instructor. Cover this grid with a piece of acetate (or use the laminated grid, or insert the grid into the plastic sleeve.)
Obtain a set of chemicals (4 plastic transfer pipets and a microcentrifuge tube).
For each column, place a drop of the liquid or a piece of the solid on the acetate sheet in the area indicated by the column label at the top of the reaction grid. Do not place any material on the darkened areas.

!!!Click here to See Movie.
For each row, place a drop of the liquid or a piece of the solid on the acetate sheet in the area indicated by the row label at the right side of the reaction grid. (Avoid contamination of the droppers. Wipe the forceps if they touch a solution.) Make careful observations, and record them in the data table provided. Follow this procedure to make systematically all of the possible combinations.

!!!Click here to See Picture.
!!!Click here to See Picture.
Wash hands.

Questions

How confident are you of the observation made when the blue liquid is placed on the shiny metal?
Identify the chemicals used that react with two or more other chemicals.

Makeup Additional Question

H1. Several students, who received chemicals which appeared to be the same(3 colorless solutions, a metal, and a blue solution(B)), observed the following. Can you explain the differences?

!!!Click here to See Picture.

Handout Makeup

Name ___________________________ Class _______

Teacher __________________________

SmallScale 013 Observing Reactions

H1. Several students, who received chemicals which appeared to be the same(3 colorless solutions, a metal, and a blue solution(B)), observed the following. Can you explain the differences?

Use the pictures above to answer this question.

Curriculum-

All students receive what appear to be identical groups of chemicals. In fact, there are two sets of look alike chemicals, and their reaction patterns are dramatically different from one another. The purpose of the activity is to get students to focus on observations and on the scientific method. It is important to work in groups. Groups of two or three persons per group work well.
In the final analysis, some students tend to disregard their observations and adopt the majority opinion. This experiment can help to build a student's confidence in making observations.

Safety-

Several of the chemicals are toxic. The vapors of several chemicals are toxic. Wear goggles and apron. Unknown chemicals may pose unexpected hazards, so use caution in dealing with them. Wash spills with water. Wash hands immediately upon contact with chemicals. Avoid breathing vapors from chemicals. Avoid ingesting the chemicals.
The toxic chemicals are hydrochloric acid, ammonia, and zinc nitrate. By controlling amounts and following the stated precautions, risks are minimized. Because this experiment usually is performed early in the course before students have had a great deal of laboratory experience, the teacher should attend very carefully to safety rules and procedures.

Time-

Teacher Preparation: 30 minutes

Class Time: 30 minutes

Materials-

Two sets of chemicals which appear to be alike. Half of the students receive one set while the other students receive the other set.
- Set One: Plastic transfer pipets labeled A to D and a microcentrifuge tube labeled E containing:
  1. 0.1 M Na₂CO₃ -- (Dissolve 1.06 g of Na₂CO₃ in enough water to make 100 mL of solution.)
  2. 0.1 M copper sulfate pentahydrate -- (Dissolve 2.50 g copper(II) sulfate pentahydrate (CuSO₄¥5H₂O) in enough water to make 100 mL solution.)
  3. 0.1 M CaCl₂ -- (Dissolve 1.47 g calcium chloride dihydrate (CaCl₂¥2H₂O) in enough water to make 100 mL solution.)
  4. 1.0 M NH₃ -- (Add 33 mL of 3 M NH₃ to enough water to make 100 mL of solution.)
  5. mossy zinc metal or zinc chips (in a capped microcentrifuge tube)
Set Two: (A similar group of materials).
1. 1 M hydrochloric acid solution (Add 33 mL of 3 M HCl to enough distilled water to make 100 mL of solution)
2. bromcresol green solution (0.01 g diluted with a 0.001 M sodium hydroxide solution to 100 mL)
3. 0.1 M zinc nitrate (Dissolve 2.97 g zinc nitrate (Zn(NO₃)₂¥6H₂O) in enough water to make 100 mL solution.)
4. 1.0 M NH₃ -- (Add 33 mL of 3 M NH₃ to enough water to make 100 mL of solution.)
5. mossy zinc metal or zinc chips (in a capped microcentrifuge tube)
acetate sheet (to place over data sheet, or laminated data sheet)
2 data sheets (one for under the acetate; one for student recording ). See first figure in handout.
tweezers

Disposal-

Collect the used chemicals into a single vessel. Filter unreacted zinc metal. Discard the zinc metal with ordinary solid trash. Discard the remaining liquid at the sink and flush with 10 volumes of tap water per volume of fluid.

Lab Hints-

The acetate sheet is an inexpensive spot plate. Surface tension keeps aqueous solution well puddled on the sheet's surface. There are three alternatives to using grids. The simplest is to place the acetate on the grid directly. Next is to use plastic paper holders, and slide the grid into the holder. Finally, the grid can be laminated.
If acetate sheets are used. Slide the sheet up so only one grid (half a page) is covered. Take notes on the bottom grid.
Place the solutions in plastic transfer pipets with pulled tips. Place the metal in a capped centrifuge tube.
The bromcresol green solution in set two must match the color of the copper sulfate solution in set one.

Discussion-

Use an overlay on the overhead projector in front of the class. Ask students to provide the observations. After a few minutes there may be considerable heated debate. See if two charts can be developed such that all students find their results compatible with one or the other.
The discussion can lead to ways in which scientists work out their differences. For example, do students suggest that they return to repeat experiments? Do they want to watch one another repeat experiments?

Data Table-

Set One -- Na₂CO₃ reacts with Cu²⁺ and Ca²⁺. CuSO₄ reacts with NH₃ and Zn.
Set Two -- HCl reacts with bromcresol green and Zn. (The reaction with NH₃ usually cannot be detected.) Bromcresol green reacts with NH₃, but the solution already is basic. ZnÛ± forms a precipitate in NH₃ that goes on to dissolve.

Answers-

Q1. How confident are you of the observation made when the blue liquid is placed on the shiny metal? A1. Most students will be very confident. The results are quite different, however. One turns the surface of the metal black, brown, or reddish. the other shows no reaction.
Q2. Identify the chemicals used that react with two or more other chemicals. A2. Set One: A reacts with B and C; B reacts with A, D and E. Set Two: A reacts with B and E. Students will answer for only one set.

CoopLearn-

This experiment is best performed in pair or triads. It is extremely important that time be left for groups to discuss their results.

Reference-

This experiment is modeled after Experiment A05 of the Doing Chemistry series.

Key Words 1-

observation, scientific method, evidence for reaction

Elements-

Zn Ca H Cl C Na Cu N

Handout